Agents also had reason to believe Salley, now accused of shooting a Chicago police officer, was ready for a fight. At the mother's home, the FBI said, they found an empty gun case, gas masks and a sales receipt for ammunition.

They also knew Salley had a violent past, including an alleged threat to shoot two of his children.

As the agents drew up a plan to capture Salley at the South Loop apartment of his girlfriend, Joseph M. Airhart Jr., a veteran Chicago police detective, volunteered to take the lead, police said. Airhart knocked on the door of Apartment 301 and said he had mail for Salley.

Airhart was shot in the head seconds after that door opened. As he continued to fight for his life Wednesday at Northwestern Memorial Hospital, authorities searched for answers about how an arrest strategy that comes off without a hitch most times went so terribly wrong Tuesday. Their early answer: Even textbook tactics, carried out by experienced officers, can go bad when a suspect is eager for battle.

New details of the shooting emerged Wednesday in interviews and court records, including an exchange between the detective and the suspect.

Airhart, dressed in street clothes, had followed Salley as he ran into the apartment and fellow officers reported Airhart said, "You don't want to do that" seconds before the gun battle erupted.

The FBI also said Salley had stockpiled an assault rifle, a second long gun, a shotgun, at least two large-caliber revolvers, thousands of rounds of ammunition, shotgun shells, pepper spray and night-vision goggles.

Airhart underwent three hours of surgery Wednesday morning to remove a blood clot in his brain. He was in critical but stable condition late Wednesday.

"The next 24 hours will be critical to his survival," said Dr. Hunt Batjer, chief of neurological surgery at Northwestern. "There are immediate threats to his life. That's our focus."

Documents released Wednesday show the search for Salley began near dawn Tuesday.

The 41-year-old father of six was accused in a federal complaint with the armed robbery of $239,000 at a supermarket bank branch Friday.

After a witness followed the robber and called police with his license plate number, federal agents said they traced the stolen vehicle to Salley and went to a bungalow owned by his mother at 9220 S. Dauphin Ave.

Agents found the empty gun case and other items inside, the FBI said in an affidavit.

They also questioned Salley's mother, Estella Salley. "She stated she was concerned about Salley because it seemed his `mind slipped,' the FBI said. And they found a key clue: a letter from Salley's girlfriend that bore the address 1307 S. Wabash, Apt. 301, records show.

Agents converged at the apartment building, tenants let them in, and they found the vehicle allegedly stolen by Salley.

Roughly a dozen agents, including Airhart, 45, who was assigned earlier this year to the task force, assembled in the garage. By about 10 a.m., Airhart and four other law-enforcement officers took an elevator to the third floor, the FBI said.

Using his delivery ruse, the smooth-talking Airhart knocked on the door and spoke to Salley briefly, the FBI said. Then Airhart gave his backups a signal to head for the door.

Salley ran inside, turning out of sight from the front door, as Airhart chased him with the other officers following behind and announcing they were police, the FBI said.

Police say Salley, with a .45-caliber handgun in one hand and a .44-caliber in the other, opened fire after Airhart spoke one last time.

As Airhart fell to the floor, the backup officers opened fire but had to retreat out the door as Salley continued to shoot, the FBI said. They tried to re-enter, the FBI said, but were once again forced out by gunfire without Airhart. Salley then bolted the door.

An FBI sniper outside spotted Airhart facedown on the floor, with Salley, who had been shot four times, sitting next to him and waving a gun, the FBI said.

Salley allegedly told negotiators he could shoot the officer if he wanted to, and the standoff didn't end for more than two hours.

The FBI said Salley admitted shooting Airhart during the ambulance ride and at the hospital. His motive, the FBI said, was that an IRS dispute had threatened his livelihood. He also asked why the federal government sent a "cop" to do its "dirty work," according to court records.

FBI supervisors from Washington were still conducting an inquiry into Airhart's shooting and the standoff Wednesday. But experts in police tactics said the team did everything by the book.

"The ruse was used to determine if [Salley] was there," said Chicago Police Chief of Detectives Phil Cline. "It's unfortunate, but bad guys don't play by the rules. He did something wrong--not them."